Powderless etching bath



United States Patent 3,357,921 POWDERLESS ETCHING BATH John A. Easley, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed July 6, 1966, Ser. No. 563,034 2 Claims. (Cl. 252-79.4)

ABSTRACT OF THE DISCLOSURE This invention concerns a powderless etching bath composition and method consisting essentially of impinging an etching bath composition of nitric acid, a water-immiscible organic fluid substantially stable in the presence of dilute nitric acid, and a monosulfonate-containing filming agent which is soluble in both the water-immiscible organic fluid and in the bath, the balance being water.

This patent application is a 'continuation-in-part of US. patent application Ser. No. 478,453, filed Aug. 9, 1965, now abandoned.

This invention relates to an improved method and bath for etching photoengraving plates and is more particularly concerned with an improved filming agent for powderless etching baths as employed in the production of metal photoengraving plates, name plates, templates, patterns and the like.

In the conventional method of making such objects as, for example, photoengraving plates, a fiat or cylindrical plate of an acid-soluble metal such as magnesium, zinc or one of their alloys is coated with a light-sensitive coating or enamel. This coated surface is exposed to light through a negative'having an image thereon so as to produce an image on the coating. Next, the exposed surface is developed forming an acid-resistant coating in the form of the image produced by the exposure. This acidresistant coating is generally further hardened by heating and the final acid-resistant image is called the resist." The image-bearing surface of the plate is then subjected to etching by an acid to produce an image in relief. In carrying out the etching process, techniques are employed to reduce lateral etching which undercuts the resist and/or" invention is directed.

More particularly, one facet of powderless etching to which this invention is directed is that of improving etching bath chemical stability. An essential ingredient of the etching bath is a strong acid such as nitric acid which performs the function of etching or dissolving the metal in unprotected areas of the plate. Other bath ingredients include a water-immiscible organic fluid and certain filming agents. The combination of the latter two bath ingredients functions to minimize lateral etching. It is necessary in wuss starti -353E E" Mini ilk;

carrying out the process that all of these ingredients be contained in the same bath. Thus, to some degree, chemical interaction between the etching acid and the other bath components may be a source of bath chemical instability which is broadly defined as the inability of the bath to reproduce uniform results on consecutive plates over a period of time. The ingredients in the bath of the instant invention keep this instability down to a minimum.

Another facet of powderless etching to which this invention is directed is that of improving the filming tendency of the etching bath. Upon application of the bath, it is believed that a polar segment of a hydrophobichydrophilic filming agent molecule attaches to the exposed metal surface. The hydrophobic part of this same molecule having affinity for the molecules of the waterimmiscible acid-stable organic fluid attracts such molecules to its thereby perfecting an acid-resistant film. However, it is a requirement of this combination of filming agent and water-immiscible organic fluid that it have selective filming tendencies, i.e., an acid-resistant film is formed on relief side walls while in adjacent non-resist areas such a film does not occur or if it does occur, it does not remain 'under the conditions of the baths application. Also filming selectivity is further complicated when it is applied to a photoengraving plate by the fact that there may be different kinds of image areas which require etching to different depths as, for example, in open line areas which usually require depths of etch of about 0.02 inch and line screen halftone areas in the same plate which need uniform depths of about 0.005 inch in the highlight areas.

In etching printing plates of a magnesium-base alloy, that is, alloys containing at least 80 percent of magnesium, bearing a resist of printed matter such as newspaper copy, with one of the etching bath formulations within the purview of, for example, US. Patent No. 2,846,294, viz. an etching hath made up of dilute nitric acid, a wetting agent of tetradecyl sulfate and a water-immiscible liquid petroleum fraction boiling in the range from to 390 C., a definite shortcoming exists. This shortcoming is that the etching action will sometimes cease in the small bare areas of the plate encircled by a line of resist, as in the letters a, A, b, B, d, D, e, g, o, p, P, Q, which may be referred to as closed letters, and in the narrow spaces between adjacent portions of the resist, after only a moderate depth of etch is produced. This holds true in spite of continued exposure of the plate to the etching bath which will continue to etch the larger areas of the bare portions of the plate, that is the areas around the letters, leaving them in relief. The depth of the etch produced in the areas encircled by a line of resist before the etching action comes to a stop will be more or less directly proportional to the size of the encircled areas. That is to say, the smaller the bare encircled areas subjected to the etching action of the bath, the sooner the etching action ceases. In some instances, the cessation of the etching action of the bath within the encircled areas results in an insufiicient depth of etch being produced in these areas. This is the case even though within the same time of exposure to the bath an adequate etching depth is readily obtained in the adjacent larger areas, namely, those beyond the immediate vicinity of the letters, where the etching action does not become prematurely arrested or retarded. Further research has resulted in the discovery that by using a filming agent such as that defined in the instant invention the etching action of the bath is caused to continue in areas in which the 3 etching action would otherwise cease. As a result, a wider range of letter sizes can be clearly etched in relief to a sufficient depth within and adjacent to resist encircled areas for printing purposes.

A principal object of this invention is to provide a relatively stable filming agent which is soluble in the etch bath in the proportions used and method for its use in powderless etching baths. It is tr further object to provide a chemically stable etching bath capable of producing uniform and proper depths of etch in all parts. of combination photoengraving plates. Still another object is to provide a powderless etching bath capable of producing desired depths of etch in name plates, metal patterns and templates. Another object of the present invention is to provide a powderless etching bath which may be intermittently used over a period of several days. An additional object is to provide a powder-less etching bath capable of use in a single continuous application (one shot baths) using combination type photoengraving plates. Other objects will become apparent hereinafter.

Such objects can be accomplished by an etching bath comprising (A) nitric acid; (B) Water-immiscible organic fluid substantially stable in the presence of dilute nitric acid; (C) the acid form of a monosulfonate-containing ester filming agent (agent being defined as a compound or a blend of compounds) which is soluble in both the water-immiscible organic fluid and in the bath in the proportions used as may be characterized by the formula:

II soda-m-o-o-m wherein R is a member selected from the group consisting of saturated and unsaturated, branched and straight chained, aliphatic hydrophobic radicals having a continuous chain of about 16 to 20 carbon atoms, including the carbonyl carbon atom, and which may contain up to two hydroxyl substitutions, and R is an organic hydrocarbon radical containing from 1 to 12, inclusive, carbon atoms, and (D) balance is water.

These bath components and the amounts required are described in more detail below. Since etching baths are usually made up to a stated volume, the amounts of bath ingredients, for convenience, are expressed in grams per liter of bath. Unless otherwise specified, amounts of nitric acid are on the basis of 100 percent nitric acid.

The acid ingredient is generally nitric acid, although mixtures of nitric acid and small amounts of sulfuric, hydrochloric or acetic may be operable in some baths. Operable amounts of nitric acid range from 30 to 200 grams per liter of bath, but a preferred range is from 50 to 150 grams per liter of bath. A desired range is 60 to 140 grams per liter of bath.

A second ingredient is an organic, water-immiscible fluid which may be a single compound or blend of such compounds. Required properties of this additive are that it be substantially stable in the presence of dilute nitric acid, at bath temperatures, and that it have some solvency for the filming agent. Substantially stable is defined as non-occurrence of deterioration, within a reasonable time period, due to the presence of dilute nitric acid which significantly alters the function of the organic waterimmiscible component within the bath in an unfavorable manner. Also'it is essential that this additive is a liquid at bath temperatures. Suitable organic materials which can be used individually or in combination are hydrocarbon aromatics, aliphatics and naphthenics boiling from 90 to 390 C. as, for example, ligroin, kerosene, gas oils, diethylbenzenes, tetramethylbenzenes, diisopropylbenzenes, and dodecylbenzene. Other examples of water-immiscible fluids include turpentine, monochloroethylbenzene, ethylbutyl ketone, isophorone, methylhexyl ketone, d-limonene, diisodecyl phthalate, dicapryl adipate, and

the like. Generally, it may be said that water-immiscible esters, ketones, terpenes, ethers, aliphatic, naphthenic and aromatic hydrocarbons are operable. Certain commercial solvents also serve very effectively as this com- 5 ponent of the bath. An example is a commercial aromatic solvent under a trade name of Penola H.A.N. which contains 84 percent aromatics, has a flash point of 140 R, an aniline point of minus 2 F., and a distillation temperature range at 760 mm. Hg: initial boiling point of 340 F., 50 percent distilled at 446 F., and dry point at 532 F. Another commercial aromatic solvent employed has the trade name Solvesso 150 (SC-150) which is a mixture of approximately 90 percent alkyl benzenes, 2 percent naphthalene and 8 percent naphthenes. It has a flash point of 150 R, an aniline point of minus 18" F., and a distillation temperature range at 760 mm. Hg: initial boiling point of 303 F., 50 percent distilled at 378 F., and dry point at 415 F. The amounts of the water-immiscible organic fluid employed may range from 3 to 150 grams per liter of bath but a preferred range is from 5 to 100 grams per liter of bath. A desired range is 10 to 60 grams per liter of bath.

A third ingredient is the monosulfonate-containing organic filming agent (agent being defined as a compound or blend of compounds) as may be characterized by the formula:

o som-m-ii-o-m wherein R is a member selected from the group consisting of saturated and unsaturated, branched and straight chained, aliphatic hydrophobic radicals containing 16 to 20 carbon atoms, including the carbonyl carbon atom, in continuous carbon-carbon bonds and which may contain up to two hydroxyl substitutions, and R is an organic hydrocarbon radical containing from 1 to 12 carbon atoms, inclusive. This filming agent has been found to he more stable during storage and in the bath than sulfated castor oils.

It is unimportant as to what hydrogen displaceable ion said filming agent contains prior to its being converted into the acid form in the bath. Suitable materials that said filming agent can contain as its hydrogen displace- -able ion in addition to hydrogen itself, are the alkali metals, alkaline earth metals (defined as including magnesium), ammonium and substituted ammonium radicals. However, heavy metal salts, such as salts of mercury or lead are undesirable. Generally, it is preferred that the filming agent molecule be a monosulfonate but in some instances, higher sulfonates may be present in some mix- 55 tures of the filming agent in a minor proportion, the

major proportion in all cases being monosulfonate. Location of the sulfonate group on any particular carbon atom in the R member of the above formula determines to some extent the filming characteristics of the filming 60 agent, but is not otherwise known to be critical.

A critical overall solubility limitation on the filming agent of this invention is that it must, first of all, be soluble in the bath and secondly, soluble in the water-immiscible organic fiuid and thirdly, partially soluble (at least 0.01 percent of the total solute) in the aqueous acid solution.

The foregoing sulfonic materials also have a high stability in the presence of strong acids and are to be distinguished from the sulfated compounds of United States Patents 2,828,194 and 2,846,294. The manner of preparation of the sulfonates, the chemical bonding, and as to be expected, the chemical properties are critically different. Illustrative of the differences in the chemical bonds are 75 the following equations which indicate a carbon to sulfur bond for the sulfonates and a carbon to oxygen bond for the sulfates:

The amounts of the filming agent employed can range from 0.2 to 20 grams per liter of bath but a preferred range is from 1 to 15 grams per liter of bath. A desired range is from 2 to grams per liter of bath. An example of a monosulfonate containing filming agent suitable for use herein is sodium isopropyloleate sulfonate. Additional instances of filming agents appear hereinafter as Examples 1 through 10.

The remaining essential bath component is water, which makes up the balance of the bath, but it should be understood that other materials may be present in the bath since it is not practical to employ the foregoing components in a pure state. Also, additional materials may be added to enhance a particular effect.

In the practice of the present invention, it has been found desirable to impinge the bath upon the surface to be etched as by spraying or by splashing. The bath selectively forms an acid-resistant film on the plate thereby allowing etching in resist-free areas normal to the surface of the metal plate while, at the same time, protecting the resist and the relief side walls from lateral etching.

In formulating the etching bath, it has been found to be a general rule that if the concentration of the nitric acid is increased within the aforementioned limitations, it is necessary to also increase the proportion of filming agent employed. However, due to the fact of bath depletion as the etching process is carried out, amounts of components are designated in terms of initial concentrations.

Suitable metals which may be etched by the process of this invention include zinc, zinc base alloys, magnesium and magnesium base alloys all of which are substantially homogeneous metals suitable for photoengraving or the like. A zinc or magnesium base alloy is defined as having at least 80 percent of the base component. The average bath temperature may be within a range from 40 to 120 F. but a preferred range is from 60 to 90 F A preferred embodiment of this invention comprises nitric acid in an amount of about 60 to 130 grams per liter of bath, a water immiscible-organic fluid in an amount of from about 10 to 70 grams per liter of bath and the acid form of a filming agent which is soluble in the water immiscible-organic fluid and in the bath in the proportions used, in an amount from about 1 to 7 grams per liter of bath. The remainder of the mixture is water. To carry out the etching process, it is preferred to employ an etching machine of the type disclosed in United States Patent 2,669,048, issued Feb. 16, 1954. In this Dowetch machine rotating elongated paddles intermittently splash the etching bath upwardly against the image-bearing side of the object being etched. The splashing action of the paddles also serves to keep the bath in a homogeneous state. A bath of this invention applied in the foregoing manner will produce a good combination photoengraving plate having depths of etch in open line areas of about 0.02 inch and up to about 0.0065 inch in 65 line screen halftone areas with minimal amounts of lateral etching in all areas. Also, comparable results are achieved when this bath is used to etch name plates and metal patterns. In

' order to produce satisfactory results etch factors in open line areas can normally be no lower than about 10 but should be in a range from about to 50. The term etch factor as used herein is defined as the ratio of the depth of etch adjacent to a line of resist divided by /z of the loss in width of metal immediately beneath the resist. It is obviously desirable to have as high etch factors as possible for faithful reproduction of the image in relief. However, it should be noted that the etch factor can be sensitive to changes in depth and therefore it 6 should be considered as only one rough approximation of plate quality.

The following examples are intended to be merely illustrative of the invention and should not be construed as limitations on the scope of the invention.

EXAMPLE 1 A powderless etching bath of 6 liters was prepared in a miniature Dow Etch machine by adding 1020 grams of 42 Baum nitric acid, 60 grams of diamylnaphthalene and 20 grams of a commercial surfactant which contained 10 grams total active ingredient consisting of sodium normal propyloleate sulfonate. The remainder of the bath was water. The addition of the filming agent was conveniently accomplished by first dissolving or colloidally dispersing it in a small amount of water and then adding the resulting solution to the bath. Bath temperature was adjusted to about room temperature, approximately F. A plate approximately 2" x 3" x .064" of magnesium base alloy containing 3 percent aluminum, 1 percent zinc and trace amounts of impurities having a polyvinyl alcohol resist thereon was descummed by brushing the surface with dilute nitric acid until the surface was bright. The plate was then placed in the machine, the paddles turned on, and etching allowed to continue for 8 minutes. The resulting plate had a depth of etch in open line areas of about 0.0145 inch and in 65 line screen halftone areas a depth was about 0.0055 inch. The general appearance of the plate was very good and the etch factor in the open line areas was approximately 30.

EXAMPLE 2 In a manner similar to that of the foregoing example, a 6 liter etching bath was prepared from 678 grams of pure nitric acid (1020 grams of 42 B. nitric acid) 60 grams of diethylbenzene (a mixture of the isomers) and 1.5 grams of sodium normal propyloleate sulfonate. The remainder of the bath was water. Bath temperature was adjusted to 70" F., and a rolled zinc photoengraving plate, 1%" x 3" x .064", having a nominal composition of about 99 percent zinc with a balance of aluminum and magnesium, was etched for 8 minutes. The depths of etch achieved were about 0.012 inch in open line areas and about 0.005 inch in 65 line screen halftone areas, producing thereby a plate suitable for printing.

EXAMPLES 3 THROUGH 7 Similarly as in Examples 1 and 2, a number of 6 liter powderless etching baths were prepared using various filming agents and solvents in accordance with the present invention. These baths were used to etch sample plates of zinc and of magnesium photoengraving grade metal. All of the sample plates were of the same (1%" x 3" x .064) and had a test image combination of lines and halftones. Each plate was numbered serially in the order of etching in each bath. The nitric acid component of each bath was replenished (after each sample was etched) at the rate of 1 ml. of 42 B. HNO per gram of zinc dissolved, or, 5 ml. of 42 B. HNO per gram of magnesium dissolved. During the etching of plates prior to that given as an example in the following table, the bath composition in each case was adjusted to the specified composition in the table below.

The table presents the results obtained from etching said sample plates using the present powderless etching baths. The major bath ingredients as well as the conditions 0 under which such etching was accomplished are also shown. Results of each test were 'by visual inspection evaluated as very good (VG), good (G), fairly good (PG), and fair (F). All the baths so-tested were considered to have produced plates having at least some commercial value.

Filming agent designation for Examples 3-10 (A) Amyl ester of sodium salt of sulfonated oleic acid (commercially available as Sul-fon-ate OE-SOO) 97% active.

by comparable results. Examples of these filming agents are sulfonated methyl, ethyl, propyl, butyl, valeryl, hexyl,

(B) Isopropyl ester of sodium salt of sulfonated oleic 5 acid. 100% active.

(C) Same as B except 62.8% active). (D) Hexyl ester of sodium salt of sulfonated oleic acid. 61% active.

(E) Z-ethylhexyl ester of sodium salt of sulfonated l oleic acid. 100% active.

(F) Same as E except different sample lot. (G) Z-ethylhexyl monosodium salt of sulfonated oleic acid. 100% active.

Example 20 Filming Agent Component. A A B C Amount 1 (gms.) (Total) 57.3 31.3 36 7. 5 Metal (Photocngraving grad Mg Zn Mg Zn Etching Conditions:

Bath Temperature, F 70 75 74 75 Etch Time (minutes) 12 6. 5 12 Paddle Speed (r.p.m.) 620 700 600 700 5 HNO: 00m onent: 2 42 Be. 846 1, 270 846 1, 185 Pure (gins).-- 583 875 583 816 Solvent Componen SC-150 (ml.) 60 19. 2 180 19. 2 Dodecylbenzene 1 (ml.) 19. 2 18 19. 2 PenolaH.A.N.(1n1.). Panasol AN-l 9.5 9. s Results:

Line Depth (inches) 013 022 Halitone Depth (inches) 0035 0025 Etch Rate (incheslmlnut 002 0019 Side Etch (inch .0012 0006 Etch Factor 4 10. 8 3 7 Ap earance of Plate..-. Fair Fair Plate umber 12 3 1 Active portion only. Dodecylbenzene (See Col. 3, line 72, supra). Penola Heavy Aromatic Naphtha (See Col. 4, line 6, supra).

A group of relativelv non-toxic low volatile, aromatic naphtha made by Amoco Chemicals Corp., having a wide boiling range and typically 40 containing 99 percent high boiling aromatics.

See Col. 5, lines 68 and 72 for definition, supra.

Example Filming Agent Component D E F G Amount 1 (ms) (Total) 12 18 6 66 Metal (Photoengraving grade) Zn Zn Mg Zn Etching Conditions:

Bath Temperature, F 64 68 70 80 Etch Time (minutes) 12 12 10 16 Paddle Speed (r.p.rn.) 700 750 620 650 HNO; Component:

24 270 44. 4 Dodecylbenzene L-.. 24 44. 4 Panasol AN-l B 18 12 22. 2 Results:

Line Depth (inches) 0195 021 022 025 Helitone Depth (inches)-. .0035 .005 0075 .004 Etch Rate (inches/minutes). 0017 .0018 0022 .0017 Side Etch (inches) .001 .0008 .001 .0014 Etch Factor 4 19. 5 26 22 1s Appearance 01 Plate F. Good Good V. good good Plate Number 9 11 3 23 1 Active in portion only. 1 Same as in previous Examples 1 through 4. Panasol AN-l-same as in previous examples. See Col. 5, lines 68 and 72 for definition, supra.

In a manner similar to that of the foregoing examples the following filming agents may be substituted for the aforementioned specific filming agent accomplishing there- 7 heptyl, octyl, and like esters of stearic, a-hydroxystearic, a,fi-dihydroxy stearic, oleic, ricinoleic, linoleic, linolenic, and the like acids, for example sodium normal propyloleate sulfonate.

Also, in a similar manner to that of the foregoing examples, other objects such as name plates, templates and metal patterns may be etched with the powderless etching baths of this invention achieving thereby a quality of etch comparable to that shown for photoengraving plates shown above.

Various modifications may be made in the etching bath additives of this invention and the method of applying the bath without departing from the scope of this invention, and it is to be understood that I limit myself only as defined in the appended claims as read in the light of the specification.

What is claimed is:

1. An etching bath consisting essentially of (A) 30 to 200 grams per liter of bath of nitric acid; (B) 3 to grams per liter of bath of a water-immiscible organic fluid substantially stable in the presence of dilute nitric acid; (C) 1 to 15 grams per liter of bath of a monosulfonate-containing filming agent which is soluble in both the water-immisible organic fluid and in the bath in the proportions used characterized by the formula wherein R is a member selected from the group consisting of saturated and unsaturated hydrocarbon hydrophobic radicals containing about 16 to 20 carbon atoms, including the carbonyl carbon atom, in continuous carboncarbon bonds and which may contain up to two hydroxy substitutions, and R is an organic hydrocarbon radical containing from 1 to 12 carbon atoms, inclusive; and (D) the balance being water.

2. A method for etching photoengraving combination acid-soluble metal plates selected from the group consisting of zinc, zinc base alloys, magnesium and magnesium base alloys which comprises impinging an etching bath upon the metal surface to be etched, such bath consisting essentially of (A) 30 to 200 grams per liter of bath of nitric acid; (B) 3 to 150 grams per liter of bath of a water-immiscible organic fluid substantially stable in the presence of dilute nitric acid; (C) 1 to 15 grams per liter of bath of the acid form of a monosulfonate-containing filming agent which is soluble in both the water-immiscible organic fluid and in the bath in the proportions used characterized by the formula:

ii SOaH-Rr-C-O-RI wherein R is a member selected from the group consisting of saturated and unsaturated hydrocarbon hydrophobic radicals containing about 16 to 20 carbon atoms, including the carbonyl carbon atom, in continuous carboncarbon bonds and which may contain up to two hydroxyl substitutions, and R is an organic hydrocarbon radical containing from 1 to 12 carbon atoms, inclusive; and (D) the balance being water.

References Cited UNITED STATES PATENTS 2,846,294 8/1958 Patterson et al 252-795 2,857,370 lO/l958 Patterson et a1 252161 2,979,387 4/1961 Eady et a]. 25279.5

LEON D. ROSDOL, Primary Examiner.

M. WEINBLATT, Assistant Examiner. 

1. AN ETCHING BATH CONSISTING ESSENTIALLY OF (A) 30 TO 200 GRAMS PER LITER OF BATH OF NITRIC ACID; (B) 3 TO 150 GRAMS PER LITER OF BATH OF A WATER-IMMISCIBLE ORGANIC FLUID SUBSTANTIALLY STABLE IN THE PRESENCE OF DILUTE NITRIC ACID; (C) 1 TO 15 GRAMS PER LITER OF BATH OF A MONOSULFONATE-CONTAINING FILMING AGENT WHICH IS SOLUBLE IN BOTH THE WATER-IMMISIBLE ORGANIC FLUID AND IN THE BATH IN THE PROPORTIONS USED CHARACTERIZED BY THE FORMULA 